1. Field of the Invention
The present invention mainly relates to a multicarrier transmitting method and its transmission circuit used for a cellular base station.
2. Related Art of the Invention
Recently, digital mobile communications have made remarkable progress, and the arrangement of the infrastructure including a base station is urgently required. Especially in large cities, a smaller base station is necessary for use in a difficult place for radio communications such as an area encompassed by large buildings, an underground street, etc. Thus, a smaller base station device replacing the conventional large base station device is requested.
The conventional multicarrier transmitter circuit will be described below by referring to FIG. 4. In FIG. 4 showing a block diagram of the conventional multicarrier transmitter circuit, an exchange 402 retrieves only a necessary signal from among signals for respective users transmitted by a public telephone network 401, and the signal is output to each of n channels. The n output signals are handled in an appropriate base band process such as a band limit filtering process by base band process circuits 403-1 to 403-n, modulated by modulators 404-1 to 404-n, and added up by an addition circuit 405 in an analog system. An output is amplified by a high frequency power amplification circuit 406, and is transmitted through an antenna 407.
FIG. 5 shows a conventional common multicarrier transmission signal. FIG. 5(a) shows the frequency spectrum of a common multicarrier transmission signal. FIGS. 5(b), 5(c), 5(d), and 5(e) shows the rotation of the vector of one carrier. FIG. 5(f) shows a case in which four carriers have 90-degree phases each other. FIG. 5(g) shows a case in which vectors of the carriers are composed. FIG. 5(h) shows a change of total power of a multicarrier transmission signal with time.
As shown in FIG. 5(a), each carrier is positioned with its frequency shifted from one another so that the central frequency f1, f2, . . . , fn cannot be superposed. The vector of one (f1) of the carriers is rotating counterclockwise from the stating point shown in FIG. 5(b). After xc2xc period, it is rotated to the position shown FIG. 5(c). After xc2xd period, it is rotated to the position shown in FIG. 5(d). After xc2xe period, it reaches the position shown in FIG. 5(e). After a period, it reaches the original point shown in FIG. 5(b). Since the central frequency of the carrier of a multicarrier is shifted little by little, the value obtained by composing each vector changes with time although they have an equal carrier amplitude.
Assuming that there are carriers f1 to fn having four different frequencies and an equal amplitude, and they are shifted by 90 degrees at a certain moment respectively as shown in FIG. 5(f), f1 and f3 have an equal value in the opposite directions, f2 and fn have an equal value in the opposite directions, and the composite vector reached nearly 0. If the four vectors are arranged in the same direction at a certain moment, the composite vector quadruples. For example, if there are three carriers, that is, f1, f2, and f3, as shown in FIG. 5(g), the composite vector is f which changes with time because respective angular speeds are different a little bit from one another. Therefore, the total power of the transmission signal changes with time, and a peak power at a level considerably higher than that of an average power is occasionally generated as shown in FIG. 5(b).
Furthermore, a present portable telephone in a Code Division Multiple Access (CDMA) system has been developed to replacing the current digital portable telephone by reserving a larger communications capacity. Since the CDMA is described in detail in xe2x80x98CDMA System and New Generation Mobile Communications System (edited by Akira Ogawa, in a Trikeps series; Chapter 1 PP12-25; published by Trikeps Ltd.xe2x80x99, the detailed explanation of the system will be omitted here. Since a base station of the digital portable telephone uses a linear modulation system, and a signal is transmitted along a plurality of carriers (multicarrier), a transmission and reception circuit requires strict linearity and a wide dynamic range.
FIG. 6 is a block diagram of the main part of a multicarrier transmission circuit in the conventional CDMA system. In FIG. 6, kxc3x97n channel signals retrieved from a public network through an exchange (not shown in the attached drawings) are input to channel input terminals 601-(1-1) to 601-(n-k), and multiplied respectively by code multipliers 602-(1-1) to 602-(n-k) using a code selected by a code selection circuit 607. The resultant k outputs are added up by using digital addition circuits 603-1 to 603-n, and obtains n outputs. Using the resultant n outputs, the n carriers generated by carrier generators 605-1 to 605-n are modulated by modulators 604-1-604-n. The resultant n outputs are added up in an analog system by an addition circuit 606, and a multicarrier signal is obtained at an output terminal 608. The signal is amplified by a high frequency power amplification circuit as shown in FIG. 4, and transmitted through an antenna. Especially, a transmission circuit has a circuit for handling high power such as a power amplification circuit, etc., and is designed to cover momentary maximum output (peak) power for average output power to maintain the linearity. Furthermore, since a high transmission rate is required to obtain a larger communications capacity, the bandwidth of a transmission signal ranges from several MHz to tens of GHz. Therefore, it is necessary to use a circuit operable with a signal which is variable by {fraction (1/10)} microsecond.
However, when a ratio of momentary maximum output power to average output power (peak factor) becomes high, the transistor of a power amplification circuit also becomes large, thereby requiring average power obtained by considerably lowering the output level down from a saturated output power. In this manner with the level lowered, the ratio of the DC supply power of a power amplifier to the retrieved transmission power (power conversion efficiency) is deteriorated. Especially, in the CDMA system, the peak factor doubles more than the value in the conventional TDMA system. Furthermore, by multiplexing a code, which is a feature of the CDMA system, the peak factor becomes larger, and the peak factor of about 13 dB can be obtained at the maximum multiplexing operation. If it is furthermore multiplexed through a multicarrier, the peak factor becomes the larger as described above. Therefore, a transmission circuit such as a power multiplication circuit requires strict linearity as compared with the conventional system, and it is necessary to use an element capable of outputting power more than ten times the actual power. As a result, the transmission circuit is larger, and it is difficult to design a small base station.
To reduce a peak factor, a multicarrier transmitter circuit under feedback control as disclosed by the Japanese Patent Laid-Open No.8-274734 and No.8-818249 has been suggested. In these circuits, the signal fluctuation period is higher than tens of microseconds when a narrow band (several kHz to several hundreds of kHz) is transmitted, thereby successfully performing the process through the circuit. However, for a broad band signal of several MHz to tens of MHz, the circuit cannot follow the fluctuation period. Therefore, it is difficult to apply the transmittion the circuit used for a base station.
To solve the above mentioned problems of the conventional multicarrier transmitter circuit, the present invention aims at providing a multicarrier transmitting method and a multicarrier transmitter circuit for reducing momentary maximum output power to a small value for a broad band signal of several MHz to tens of MHz, and reducing a peak factor of a multicarrier transmission signal, thereby reducing the power of an electric amplifier, improving the power conversion efficiency, and realizing a smaller circuit.
The 1st invention of the present invention is a multicarrier transmitting method, comprising the steps of:
inputting n (n is an integer equal to or larger than 2) input signals;
generating carriers respectively corresponding to the n input signals;
modulating the carriers into n modulated signals by the input signals;
generating at least one additional signal having a frequency outside a band of the n modulated signals;
adjusting a level and a phase of the generated additional signal;
outputting a multiplexed signal by adding up the n modulated signals and the adjusted additional signal; and
amplifying the multiplexed signal, and then removing the additional signal,
wherein said level and phase of the additional signal are adjusted such that, after predicting in advance a change of a composite vector of the n modulated signals based on an amplitude and a phase of the n carriers,
a composite vector obtained after an adding operation can be lower than that before the adding operation when an absolute value of a prediction result exceeds a predetermined level.
The 2nd invention of the present invention is the method according to 1st invention, wherein:
a feed forward circuit is used as a high frequency power amplification means of amplifying the multiplexed signal; and
said additional signal is a pilot signal generated by distortion adjusting pilot signal generation means of said feed forward circuit.
The 3rd invention of the present invention is the method according to 1st or 2nd inventions, wherein said input signal is a signal in a code division multiple access system.
The 4th invention of the present invention is the method according to any one of 1st to 3rd inventions, wherein said additional signal is outside a band of the n modulated signals by 5% of a width of the band.
The 5th invention of the present invention is the method according to any one of 1st to 3rd inventions, wherein said additional signal exists outside the band of the n modulated signals both at lower and higher frequency side.
The 6th invention of the present invention is a multicarrier transmitter circuit, comprising:
input terminals for inputting n (n is an integer equal to or larger than 2) input signals;
n carrier generation means of generating n carriers corresponding to the signals input to said n input terminals;
n modulation means, connected to an output terminal of each of said carrier generation means and each of said input terminals, for modulating the carriers by the each input signals and outputting n modulated signals;
additional signal generation means of generating at least one additional signal having a frequency outside a band of the n modulated signals;
at least one variable means of adjusting a level and a phase of the additional signal generated by said additional signal generation means;
control means of controlling said variable means based on the phase and the level of each of said n carriers;
addition means of connecting an output terminal of said n modulation means to an output terminal of said variable means, and outputting a multiplexed signal by adding up the n modulated signals and the adjusted additional signal;
power amplification means of amplifying the multiplexed signal multiplexed by said addition means; and
filter means, connected to an output terminal of said power amplification means, for suppressing the additional signal from the output terminal of said power amplification means,
wherein said control means controls a level and a phase of the additional signal by using said variable means such that, after predicting in advance a change of a composite vector of the n modulated signals based on a phase and a level of each of the n carriers, a composite vector obtained after an adding operation can be lower than that before the adding operation when an absolute value of a prediction result exceeds a predetermined level.
The 7th invention of the present invention is the circuit according to 6th invention, wherein:
said power amplification means comprises a feed forward circuit;
said additional signal generation means can be a distortion adjusting pilot signal generation means of said feed forward circuit; and
said additional signal is the distortion adjusting pilot signal.
The 8th invention of the present invention is the circuit according to 6th or 7th inventions, further comprising:
m (m (=kxc3x97n) is an integer equal to or larger than n) code modulation means of modulating m preprocessing input signals into m preprocessing modulated signals by corresponding codes respectively;
code selection means of selecting the code for each of the preprocessing input signals; and
n preprocessing addition means of generating the n input signals by adding the m preprocessing modulated signals for k signals,
wherein n outputs of said preprocessing addition means are input signals to said input terminal.
The 9th invention of the present invention is the circuit according to any one of 6th to 8th inventions, wherein said additional signal is outside a band of the n modulated signals by 5% of a width of the band.
The 10th invention of the present invention is a the circuit according to any of 6th to 8th inventions,
wherein said additional signal exists outside the band of the n modulated signals both at lower and higher frequency side.
With the above mentioned method and configuration according to the present invention, the phase of each carrier of a multicarrier at a certain time point is fetched and if it is predicted, based on the phase and the frequency of each carrier, that an absolute value of a composite vector generated by a transition of the phase relation between carriers exceeds a predetermined value, then the phase and the intensity of an additional signal or a pilot signal are controlled in the direction of reducing the total vector at the time point, to reduce the absolute value of the total vector processed by a high frequency power amplifier, and thereby the apparent transient power is lowered, thereby lowering the peak factor. Thus, the required power of the power amplifier can be smaller, and the power conversion efficiency can be improved, thereby realizing a smaller circuit.